Ultrafast 2D IR vibrational echo chemical exchange spectroscopy,
akin the 2D
NMR methods, is applied to the study of dynamics of weakly
hydrogen bonded
solute-solvent complexes in liquid solutions. The strengths of the
solute-solvent hydrogen bonds are adjusted by modifying the chemical
structures of the solutes and solvents. For the eight samples
studied, the
formation enthalpies vary from -0.6 kcal/mol to -2.5 kcal/mol,
and the
dissociation time constants vary from 3 ps to 32 ps. The
dissociation rates
of the hydrogen bonds are found to be strongly correlated with their
formation enthalpies. The correlation can be described with an
equation
similar to the Arrhenius equation. As another example of chemical
exchange
spectroscopy, the rate of carbon-carbon single bond rotational
isomerization
of an ethane derivative in room temperature liquid solution is
measured.
Based on the experimental results and density functional theory
calculations, the time constant for the ethane internal rotational
isomerization under the same conditions is about 12 ps. [Preview Abstract]

The theory of the Overhauser Effect in liquids is well established, but
measured DNP enhancements from nitroxide radicals depart significantly from
prediction. To achieve large signal enhancements, milli-molar concentrations
of radicals are needed, a regime where Heisenberg exchange of the unpaired
electron is significant. Therefore, the three electron transitions resulting
from hyperfine interactions with the $^{14}$N nucleus cannot be treated as
independent in a DNP experiment. Furthermore, the relaxation rate of
$^{14}$N can be easily on the same order of magnitude or even greater than
the relaxation rate of the unpaired electron, contributing to the mixing of
the hyperfine states, even in the absence of Heisenberg exchange. We present
a quantitative study and a new model of $^{1}$H DNP enhancement of water by
varying radical concentrations and solvent viscosities of natural abundance
$^{14}$N versus $^{15}$N isotope enriched 4-Oxo-TEMPO free radical at 0.35
Tesla. [Preview Abstract]

Nitramine energetic materials (RDX, HMX and CL20) have broad applications as
explosives and fuels. Model systems (1,4-dinitropiperazine, nitropiperidine,
nitropyrrolidine and DMNA) have similar molecular structures, but they are
unable to be used as fuels and explosives. To elucidate the difference
between them, both nanosecond and femtosecond mass resolved excitation
spectroscopy have been employed to investigate the mechanisms and dynamics
of the electronically excited photodissociation of these materials. NO is a
dominant dissociation product. Based upon the experimental observation and
calculations of potential energy surfaces for these systems, we suggest that
energetic materials dissociate from their ground electronic states after
relaxing from the first excited states, and that the model systems
dissociate from their excited state. In both cases a nitro-nitrite
isomerization is part of the reaction mechanism. Parent ions of DMNA and
nitropyrrolidine are observed in fs experiments. All the other molecules
generate NO as a product even in fs time regime. [Preview Abstract]

Time resolved (fs) photodissociation experiments have been performed in
efforts to elucidate the dynamics controlling the excited electronic state
decomposition of the energetic materials RDX and HMX and their associated
model systems (dimethylnitramine, nitropyrrolidine, nitropiperidine, and
dinitropiperazine). The initial decomposition product of the energetic
materials and model systems is the NO molecule. Femtosecond pump-probe
techniques have been employed to measure the photodissociation dynamics of
these systems via the initial NO product at three wavelengths (226 nm, 228
nm, 230 nm). The NO molecule has a non-resonant two-photon absorption at 228
nm and 230 nm and single photon resonant absorption for the A$^{2}\Sigma
\leftarrow $X$^{2}\Pi $(0,0) transition. Both pump-probe transients at
non-resonant absorption and resonant absorption wavelengths indicate the
dynamics of the energetic material's decomposition from the excited
electronic state is faster than the time duration of our laser pulse (180
fs) and notably different from some of the model systems. [Preview Abstract]

Recent experimental results suggest that derivatives of
diarylethenes may be viable
candidates for switches in molecular devices. In these molecules, the
ring-opening and
ring-closing reactions are induced with laser pulses of different
frequencies. We have
shown that the ring-opening mechanism in the simplest of all
diarylethenes,
stilbene,
occurs via a HOMO-LUMO avoided crossing and subsequent
depopulation of the
excited state
with minimal involvement of other orbitals. We now show that the
same
photoinduced
ring-opening process for both oxygen ($C_{10}H_{8}O_{2}$) and
sulfur ($C_{10}H_{8}S_{2}$) containing diaryethenes involves
higher order
excited states.
We will show simulation results in which the initial laser pulse
excitation
induces
a transition from HOMO to LUMO.
Due to both nuclear motion and symmetry changes, this laser pulse
also
excites some
percentage of the electronic population out of orbitals lower in
energy than
the HOMO and into
orbitals higher in energy than the LUMO. In order for the
ring-opening
event to occur,
the electronic population in the higher excited states is first
transferred,
via a
series of avoided crossings, into the LUMO. A subsequent avoided
crossing
between the HOMO
and LUMO then allows ring-opening. [Preview Abstract]

In the past decade, cooling and trapping of atoms has allowed physicists to
probe the nature of quantum mechanics on a macroscopic scale. Molecules,
having a more complex structure, are considerably more difficult to cool.
However, it is their complex structure, including rovibrational states and
permanent dipole moments, which make them so interesting. We cool metastable
NH ($^{1}\Delta )$ radicals using supersonic expansion coupled with Stark
deceleration. The NH radicals are created by photolysis of HN$_{3}$ during
supersonic expansion. The supersonic expansion produces a cold beam of
radicals, which is loaded into a Stark decelerator. The Stark decelerator
uses time varying inhomogeneous electric fields to decelerate the NH
molecules. The resulting molecular sample has a temperature of 10 -100 mK.
Further cooling will be explored using interactions with ultracold Rubidium
atoms. [Preview Abstract]

Acephate is an insecticide that kills insects by disrupting nervous system
functions. THz spectroscopy offers a unique tool for detecting trace amount
of these materials. Using a combination of solid state first principles
simulations and gas phase quantum mechanical modeling we have studied phonon
spectra of acephate compound. This talk will present a detailed vibrational
spectra analysis over a wide range of frequency and our computational data
will be compared with available experimental results. [Preview Abstract]

We report IR-vis SFG spectra obtained at the interface of liquids
with
hydrophobic and hydrophilic solid surfaces. The hydrophilic and
hydrophobic
surfaces used were sapphire and a dense methyl-terminated
sapphire surface
from chemically bonded octadecyltrichlorosilane (OTS), respectively.
Orientation calculations of SSP (IR, vis, and SF polarizations)
and SPS
polarizations of acetonitrile on the OTS and sapphire showed tilt
angles of
approximately 90 and 40 degrees, respectively. The CD3 symmetric
stretch of
methanol (d4) at OTS and sapphire showed a blue-shift for the
latter but no
shift for the former when compared to FTIR of the bulk liquid.
This may be
due to changing H-bonding characteristics with methanol orienting
its
hydroxyl end towards sapphire and away from OTS. PPP spectra of
n-heptane
(d16) and n-hexadecane (d34) showed weaker intensity CD3
antisymmetric
stretches on sapphire compared to OTS, with a larger decrease for
n-hexadecane. This can be interpreted as the hydrocarbons curling
away from
sapphire more so than OTS, especially with n-hexadecane. [Preview Abstract]

Reactive intermediates are of crucial importance both for combustion
and atmospheric chemistry. By using our new home made Fourier
Transform
limited (10--30~MHz) Ti:Sa laser source we have probed the
vibrationless
level of the first electronic state (in the near-IR range) of both
${\rm CH}_{3}{\rm OO}$ and ${\rm CD}_{3}{\rm OO}$ radical species.
The radicals are formed inside a ${\rm Ne/He/O}_{2}/{\rm
CH}_{3}{\rm I}$
plasma created by a DC or a RF electrical discharge. The supersonic
jet expansion necessary for the rotational cooling ($\sim20\,{\rm
K}$)
is obtained by a pulsed slit nozzle ($\sim50\times0.5\,{\rm
mm}^{2}$).
The near-IR radiation, obtained by Stimulated Raman Scattering (SRS)
is injected inside a high finesse cavity. A sensitivity of the order
of $\sim20\times10^{-9}\,/{\rm pass}/\sqrt{{\rm Hz}}$ is currently
obtained. Spectrum with a resolution $\sim350\,{\rm MHz}$ for
${\rm CD}_{3}{\rm OO}$
clearly shows rotational and spin-rotation structure with effects
of the internal methyl group rotation possibly evolved. [Preview Abstract]

Knowledge of radical spectroscopy and the structure of radicals
is important
in many scientific areas, such as atmospheric systems, combustion
reactions,
biological processes, and more. Because many radicals are transient,
unstable, and generally produced in small quantities, they are often
difficult to characterize spectroscopically. In this talk, we
will present
our synthesis of theoretical and experimental data to understand the
behavior of radical photofragments.
The first part of the talk outlines our approach to understanding
vibrationally hot but electronically cold radical dynamics, with
direct
molecular dynamics and performing electronic structure
calculations using
DFT within the GAMESS package. We will then summarize our recent
development
and application of time-resolved FTIR emission spectroscopy for
the study of
photofragments. Finally, we will present a joint theoretical and
experimental investigation of the dynamics of the vinyl radical,
including
characterization of the complex interaction of rotation,
alpha-proton
motion, and anharmonic effects, and discuss their influence on
the IR
spectrum. [Preview Abstract]

We study the molecular orientations of several benzene
derivatives on large
Ag and Au clusters via first-principles calculations. We find the
lowest-energy structures, several local minima and the diffusion
barriers
for benzene, nitrobenzene, 2,4-dinitrotoluene (DNT) and
1,4-benzenedimethanethiol (BDMT). The theoretical calculations
are compared
to experimental measurements of SERS for 2,4-DNT and 1,4-BDMT on
Ag and Au
coated dielectric nanowires. [Preview Abstract]

Pure neutral (HCOOH)$_{n}$ clusters and mixed
(HCOOH)$_{m}$(H$_{2}$O)$_{n}$ clusters
are investigated employing time of flight mass spectroscopy and
single
photon ionization at 26.5 eV (from a soft x-ray laser). The
distribution of
pure (HCOOH)$_{n }$clusters is dependant on experimental
conditions. At
certain conditions a magic number is found at n = 5. During the
ionization
process, neutral clusters suffer little fragmentation because
almost all
excess energy above the vertical ionization energy is taken away
by the
photoelectron. Metastable dissociation rate constants of
(HCOOH)$_{n}^{+}$
are measured in the range of (0.1--0.8)x10$^{4}$ s$^{-1}$ for
cluster sizes
of 4$ [Preview Abstract]

A single photon of an EUV laser (26.5 eV) has enough energy to
ionize any
metal oxide cluster generated in a molecular beam. Neutral vanadium,
niobium, and tantalum oxide clusters are studied by single photon
ionization
employing a 26.5 eV EUV laser. During the ionization process,
metal oxide
clusters are virtually free of fragmentation. The most stable
neutral metal
oxide clusters under saturated oxygen conditions can be
represented as
(MO$_{2})_{0,1}$(M$_{2}$O$_{5})_{y}$ (M=V, Nb, Ta). Both O-rich and
O-deficient clusters can be observed. Oxygen-rich metal oxide
clusters with
high ionization energy are detected by 26.5 eV, but not by 10.5 eV,
ionization. For O-rich clusters M$_{x}$O$_{y}$H$_{z}$ species are
also
observed for the first time. Given these experimental
capabilities, neutral
cluster reactions and reactivity can be studied. We will present
preliminary
results of these studies. [Preview Abstract]

The new series of furazan-based energetic materials is
characterized by low
sensitivity to impact and friction. They have broad application
as fuels and
propellants; however, extra nitro functional groups attached to
the furazan
ring (e.g. 4,4$^{\prime} $-dinitro-3,3$^{\prime} $-azoxyfurazan)
adversely impact
the thermal stability of these energetic materials. In order to
evaluate the
effect of nitro functional groups on furazan-based energetic
materials the
decomposition of 4,4$^{\prime} $-diamino-3,3$^{\prime}
$-azoxyfurazan (DAAF), from
excited electronic states, has been investigated by UV excitation
(8 ns
duration) and time of flight mass spectroscopy. The NO molecule
is observed
as an initial product. Three vibronic transitions of NO are
characterized.
Simulation of the NO [A $^{2}\Sigma $ (v$^{\prime}
$=0)$\leftarrow $X
$^{2}\Pi $ (v$\prime $=1)] transition and fitting to the
intensity ratios
among NO vibronic transition yields rotational and vibrational
temperatures
of 30 K and 1265 K, respectively. Compared with NO gas spectra,
under
comparable condition, the NO from decomposition of DAAF is
vibrationally hot
and rotationally cold.
[Preview Abstract]